Substorm Features in the High-Latitude Ionosphere and Magnetosphere : Multi-Instrument Observations

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(1)Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 860. Substorm Features in the High-Latitude Ionosphere and Magnetosphere Multi-Instrument Observations BY. EVA BORÄLV. ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2003.

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(153) Acknowledgements. I want to thank my supervisor, Hermann Opgenoorth, for his enthusiasm and help and for allowing me to see distant parts of the world. I have enjoyed working in the solar-terrestrial physics group with its varying-through-time members. Of these I particularly want to thank Frédéric Pitout for his friendship and help during our time of doctorate, and Julien Verchère for being a good office-sharer. Great thanks to everybody at the Swedish Institute of Space Physics, the Finnish Meteorological Institute, and the University of Calgary (particularly Eric Donovan for inviting me there). Special thanks to Jonny Rae for his patience with IDL and radar questions. Thanks also to Jan Smedh at Ticket for organizing all my travels. Last but not least I want to thank Stu and Bat for all that makes life worth living..

(154) Contents. Acknowledgements.........................................................................................1 Introduction.....................................................................................................1 1 - Plasma and the Solar Wind........................................................................3 2 - The Magnetosphere ...................................................................................6 2.1 The Earth’s Magnetic Field..................................................................6 2.2 The Earth’s Magnetospheric Regions ..................................................8 2.3 Magnetospheric Convection...............................................................11 3 - The Ionosphere ........................................................................................14 3.1 The Auroral Oval ...............................................................................14 3.2 Ionospheric Currents ..........................................................................15 3.3 M-I Coupling and Ionospheric Convection........................................19 4 - Substorms ................................................................................................21 4.1 Observations.......................................................................................21 Growth Phase.......................................................................................22 Expansion Phase ..................................................................................25 Recovery Phase ...................................................................................30 4.2 Theories..............................................................................................31 Near-Earth Neutral line model ............................................................31 Current disruption model.....................................................................33 Storm - Substorm Relation ..................................................................34 5 - Instruments and Measurements ...............................................................36 5.1 Magnetometers...................................................................................36 5.2 Ionospheric Radars.............................................................................37 Coherent Scatter Radars ......................................................................38 Incoherent Scatter Radars....................................................................39 6 - Summary of Publications ........................................................................40 Paper I: The dawn and dusk electrojet response to substorm onset .........40 Paper II: Timing of substorm onset signatures on the ground and at geostationary orbit....................................................................................40 Paper III: The global ionospheric response to a southward IMF turning.41.

(155) Paper IV: Correlation between ground-based observations of substorm signatures and magnetotail dynamics.......................................................41 Paper V: Storm-time intense proton aurora and its relation to plasma sheet density ......................................................................................................42 7 - List of Publications not Included in Thesis .............................................43 8 - Conclusions .............................................................................................44 References.....................................................................................................45.

(156) Abbreviations. B v j E µ0 ε0 q e n AU Re eV R. β. Magnetic field (T) Velocity (km/s) Current (A) Electric field (V/m) Permeability of vacuum, 4π⋅10−7 Vs/Am Permittivity of vacuum, 8.85⋅10−12 C/Vm Electric charge (C) Elementary charge, 1.602⋅10−19 C Number density (cm−3) Astronomical unit (distance SunEarth), 149 600 000 km Earth radius, 6400 km Energy unit, 1 eV = 1.602⋅10−19 J Rayleigh, a measure of the omnidirectional emission rate in a column of unit cross section along the line of sight, where 1 R = 106 photons/s/cm2. Ratio of plasma to magnetic field pressure, nkT/(B2/2µ0), where Boltzmann's constant k = 1.38⋅10−23 J/K, and temperature T (K)..

(157) Introduction. Space physics encompasses the study of near-Earth space and our Sun, and in particular the processes in which the Sun effects the near-Earth space. This interaction can be very dynamic and can even have effects experienced from ground, such as the beauty of aurora and, maybe less appreciated, the practical implications of power failures due to electric currents in the topmost layers of our atmosphere. In this thesis I will begin with chapters dedicated to describing the space environment and the coupling between the Sun and the Earth's plasma regimes. Then I will describe observational as well as theoretical aspects on substorms, my main interest within space physics because of its dynamics and practical implications. A fundamental concept in my work has been combining data from various instruments, this in order to try to see 'the full picture', even though it certainly complicates matters. Since in my heart I am a ground-based person I will only describe a selection of ground-based instruments and their techniques in the instrumental section of this thesis, even though I have used satellite data as well in my work. The conclusions sum up the results and experiences from my four years of PhD work, and finally a summary of each publication is presented.. 1.

(158) That's the whole problem with science. You've got a bunch of empiricists trying to describe things of unimaginable wonder. Calvin and Hobbes, by Bill Watterson. 2.

(159) 1 - Plasma and the Solar Wind. Plasma is defined as an ionized and electrically neutral gas, consisting of charged particles governed by collective behavior through the generated electromagnetic fields. Plasma is the fourth state of matter besides solid, fluid and neutral gas, and in fact the most common state in the universe. It is estimated that 99% of matter in the universe exists in plasma form, of which stars are the most striking. The stars, apart from emitting electromagnetic radiation, also eject plasma, which in our Sun's case is the so-called solar wind1. The Sun more or less continuously ejects 1.6⋅109 kg/s of particles, mainly protons (H+) and electrons in roughly equal amounts. The solar wind is accelerated and moves radially out from the Sun and spreads throughout the solar system. The Sun has an interplanetary magnetic field (IMF) whose magnetic field lines, due to the solar rotation and a phenomenon called frozen-in condition (i.e., plasma carrying information on magnetic field with it), are twisted around the Sun in an Archimedean spiral, as shown in Figure 1. Average, maximum, and minimum values of the solar wind density, velocity, and magnetic field strength at Earth orbit are shown in Table 1. Minimum n (cm−3) 0.4 v (km/s) 200 B (nT) 0.2. Average 6.5 400 6. Maximum 100 900 80. Table 1: Solar wind parameter values at Earth orbit [Oulu Space Physics Textbook, http://spaceweb.oulu.fi].. 1 With a velocity of 400 km/s the solar wind takes about 4 days to reach the Earth, while electromagnetic radiation traveling with the speed of light covers the distance of 1 AU in about 8 minutes.. 3.

(160) Figure 1: The structure of the IMF carried with the solar wind [from Sakurai, 1987].. The frozen-in condition states that the magneto-hydrodynamic (MHD) Reynolds number Rm ≅ µ0σuL should be large compared to unity, which is generally true in the magnetosphere and the solar wind. In the Reynolds number, σ is the electrical conductivity, u the plasma flow speed, and L the scale size. The frozen-in concept is a consequence of plasma being highly conductive, and means that plasma on a particular magnetic field line will stay on that field line as long as the frozen-in condition is valid. Another aspect of the frozen-in phenomenon is seen when highly conducting plasma approaches a magnetic field region. The field cannot penetrate into the plasma, instead the plasma pushes the magnetic field ahead of it. If the frozen-in condition breaks down, merging and reconnection of magnetic field lines can take place. This can happen in thin boundary layers that can form between different plasma regions. In a boundary region with oppositely directed magnetic fields, as shown in Figure 2, the field lines diffuse towards the diffusion region (where Rm < 1 and consequently the frozen-in condition is no longer valid), merge with each other and then the newly merged field lines flow out on the sides of the neutral line. In the process magnetic energy is converted into particle heating and acceleration in the outflow regions. Plasma can now flow along the merged field line and 4.

(161) enter the other region. The process is called merging when fields of different origin meet, and reconnection when two merged fields return to a single source field.. Figure 2: Schematic illustration over the merging/reconnection region, where plasma and field lines flow in from top and bottom, and out on the sides [from Kivelson and Russell, 1997].. 5.

(162) 2 - The Magnetosphere. We have mentioned how the Sun's magnetic field is transported with the plasma of the solar wind toward the Earth; now we will describe how the Sun's field influences the region of space around the Earth. This region is to great extent shielded by the Earth’s magnetic field, which forms a cavity in the solar wind. However, the cavity is far from empty; rather it turns out to have a complex structure of different plasma regimes and current systems.. 2.1 The Earth’s Magnetic Field The Earth has an intrinsic magnetic field, generated in the planet's core. The mathematical description of the field is far from simple, however, the field is usually approximated by a dipolar magnetic field, as shown in Figure 3, which in first order is a reasonable approximation out to a distance of ∼10 Re.. Figure 3: Dipolar field configuration [from Brekke, 1997].. The Earth’s magnetic field can be described in a number of coordinate systems and depending on the purpose of the measurements and the history of the research, satellites and ground-based instrument networks have their 6.

(163) own favorite or even own definition of coordinates. However, a common coordinate system used when performing ground-based measurements of the magnetic field is (X,Y,Z), where X is along the geographic meridian (positive north), Y along geographic latitude (positive east), and Z vertical (positive down). Alternatively the set (H,D,Z) can be used, where H is the total horizontal field vector along the local geomagnetic meridian, declination D (sometimes referred to as E), and Z, which is the same as in the (X,Y,Z)system. These two coordinate systems are shown in Figure 4. The geomagnetic field components have the following signs: H is positive everywhere on the Earth, D is positive eastward, and Z is positive on the northern hemisphere and negative on the southern hemisphere.. Figure 4: Coordinate systems for ground-based magnetic measurements [from Kivelson and Russell, 1997].. This dipolar approximation has imaginary, so-called geomagnetic poles, on the Earth's surface. These differ somewhat from the true magnetic poles, where the observed magnetic field is vertical to the Earth's surface. It should also be mentioned that the northern pole is actually a south pole in a physical sense (as Z is positive). Finally, the geographic poles are defined by the rotation axis of the Earth. The Earth's magnetic field is not stationary, neither in time nor in space. Firstly, the term secular variation is used to describe the change of the Earth's intrinsic field with time, regarding both direction and field strength. At present the field is decreasing, so that in a few thousand years it might go down to zero or flip its direction. This behavior is however not exceptional; there is paleomagnetic evidence for reversals of the field direction throughout history. 7.

(164) Secondly, there are spatial variations of the intrinsic field, so-called magnetic anomalies. These are nearly constant in time and space, caused by local magnetic anomalies in the near-surface crust of the Earth. Usually the variations are smaller than the main field, but sometimes they can be large enough to locally double it, and in particular, anomalies can alter the declination of the magnetic field, which affects e.g. navigational instruments. Thirdly, and of interest within space physics, there are the small deviations in the field caused by external current systems in the region of space around Earth and in the upper atmosphere. In particular, the most effective currents occur at high latitudes in the topmost layer of the atmosphere and are important features in the study of magnetospheric disturbances caused by the Sun.. 2.2 The Earth’s Magnetospheric Regions At a distance of ∼10 Re from the Earth's centre the Earth's magnetic field no longer resembles the field of a dipole. The reason for this is the interaction with the Sun's magnetic field, carried with the solar wind. The interaction distorts the Earth's magnetic field lines, as the frozen-in condition does not allow mixing of different magnetized plasma regions, and confines the field lines in an extended 'bubble', the Earth's magnetosphere. The magnetosphere is pulled out on the nightside, forming a long magnetotail of unknown length. As the two different field and plasma types encounter, force balance creates a boundary between the two regions. The magnetopause acts as a magnetic barrier and shields us against solar particles. It encapsulates the magnetosphere and separates the solar wind's high-density, low-field plasma from the Earth's low-density, strong-field region. The subsolar distance to the magnetopause can be estimated by comparing the dynamic plasma pressure in the solar wind against the magnetic field pressure in the Earth's magnetic field, resulting in a location of the boundary at roughly 10 Re during calm conditions. The shape of the magnetopause is also affected by the solar wind pressure. Since both the standoff distance and shape of the magnetosphere are dependent on the solar wind conditions they are highly variable. The distortion of a magnetic field brings consequences for currents, governed by Ampère's law. ∇ × B = µ0 ( j + ε0 ∂E /∂t ). (Ampère’s law). 8.

(165) In MHD perfect charge neutrality is assumed, so for slow phenomena (plasma speed much slower than speed of light) the last term of Ampère's law is negligible compared to the first term on the right side. Ampère’s law states that strong currents will flow in particular on magnetic boundary surfaces, such as the magnetopause and the mid-plane of the magnetotail, where oppositely directed magnetic fields form a thin boundary layer. Within the magnetosphere a number of distinct plasma regions can be defined. Below we describe some of the regions, shown in Figure 5, which are of particular interest regarding the substorm process.. Figure 5: The Earth’s magnetospheric regions [from Kivelson and Russell, 1997].. The tail lobes are regions of low-density (< 0.1 cm−3) cold plasma. Their field lines map down into the polar cap in the ionosphere. Between the northern and the southern lobe is the plasma sheet, a dense and energetic plasma region. It is sometimes classified into the central plasma sheet (CPS) and plasma sheet boundary layer (PSBL). The plasma sheet maps down to the auroral zone. The CPS density ranges between 0.1 and a few cm−3, and the plasma is hot, the flow velocities being small compared to the ion thermal velocity. In the middle of the plasma sheet, where the field lines change direction from Earthward to tailward, is the neutral sheet. Due to the oppositely directed fields, the neutral sheet will carry a dawn-to-dusk current, the neutral sheet or cross-tail current. On the nightside, at an estimated downtail distance of ∼100 Re, the boundary between lobe and plasma sheet ends at the distant neutral line (DNL), where magnetic field. 9.

(166) lines which have merged with the solar wind on the dayside magnetopause are thought to reconnect again. Closer to Earth, the plasma on closed field lines co-rotates with the planet motion, constituting the plasmasphere. The radiation belts (Van Allen belts) with the westward ring current extend from 1000 km altitude to 6 Re. The region of the radiation belts contains cold, dense plasma. The ring current is carried by trapped particles, encircling the Earth by the drift mechanisms for electrically charged particles in an inhomogenous magnetic field. There is a rapid transition between dipolar and tail-like field geometry at the inner edge of the plasma sheet. The location of the inner edge is dependent on the electric field applied to the magnetosphere by the solar wind (more about this in the next section). A stronger electric field results in the boundary moving closer to Earth. During calm periods, it is located at roughly 10 Re on the nightside. Charged particles of most energies cannot drift further Earthward, however, fluctuations of the electric field causes energetic particles to enter this principally forbidden region, thereby constituting the outer radiation belt and ring current. There are several coordinate systems for describing features in the magnetosphere. Often used is the geocentric solar magnetospheric (GSM), which has X pointing from the Earth to the Sun, Y perpendicular to the Earth's dipole axis and Z along the dipole axis. Another system often used is the geocentric solar ecliptic (GSE), which has X similarly pointing from the Earth to the Sun, but Y in the ecliptic plane toward dusk and Z orthogonal to the ecliptic plane. Figure 6 shows a schematic of the XY-plane of the solarterrestrial environment.. Figure 6: Schematic of the GSM coordinate system. The X-axis points toward the Sun, the Y-axis toward dusk, and the Z-axis points out of the paper.. 10.

(167) 2.3 Magnetospheric Convection The frozen-in condition prevents any mixing of solar wind and magnetospheric particles, but field line merging and reconnection can occur when the condition breaks down locally. The result is not only particle mixing between the Earth's and the Sun's plasma populations, but also particle motion in the magnetosphere. We should however first mention another possible process, besides merging and reconnection, which can produce convection in the magnetosphere. Viscous interaction on the flanks of the magnetosphere transfers momentum from solar wind to magnetospheric flux tubes, and is estimated to account for less than 10% of the magnetospheric convection. Due to its limited importance we therefore concentrate on merging and reconnection in the following text. During northward IMF, i.e., when IMF Bz > 0, lobe merging can occur at the high-latitude magnetopause. The merged field lines are then draped over the dayside magnetosphere. Northward IMF leads to a calm magnetosphere as not much interaction between the solar wind and the magnetosphere occurs. However, during southward IMF, i.e., when IMF Bz < 0, field lines merge on the dayside magnetosphere and are subsequently pulled back by the solar wind motion, adding flux to the lobes.. Figure 7: Magneotspheric convection driven by dayside merging and nightside reconnection [from Kivelson and Russell, 1997].. 11.

(168) Figure 8: IMF By’s influence on dayside merging. The thin arrows indicate the field direction, and the thick arrows the direction where the magnetic tension will pull the newly merged field lines [from Kivelson and Russell, 1997].. The field lines which connect the Earth's and the Sun's magnetic fields are referred to as open, whereas field lines only connecting to one field source are referred to as closed. In the magnetotail, field lines convect toward the central tail and reconnect at the distant neutral line. Flux is brought back to the dayside magnetosphere on the flanks of the magnetosphere. The cycle of magnetospheric convection is shown in Figure 7. During dayside merging, not only IMF Bz effects the convection process, but IMF By is also important, as shown in Figure 8. Convection of magnetic field lines can equally be seen as an electric field influencing the plasma tied to the magnetic field lines. In the inertial system of the plasma, the electric field E is approximately zero (due to the large conductivity), and through a coordinate transformation the electric field in the magnetosphere's rest frame, Esw, is given: 0 ≈ E = Esw + vsw × BIMF. ⇒ Esw = − vsw × BIMF. 12.

(169) Here vsw is the relative velocity that the solar wind has seen from Earth and BIMF the magnetic field carried with the solar wind. Hence, during southward IMF the solar wind produces an electric field Esw in the dawn-to-dusk direction over the magnetosphere. This is called the convection electric field. Charged particles drift due to the electric and magnetic fields in the magnetosphere. The expression for the drift velocity contains terms of three origins, all caused by magnetic field topology: vD = ( E × B )/B2 + w⊥ ( B × ∇B)/qB3 + 2 w ( rc × B ) / qRcB2 The terms arise from E×B-drift, gradient drift, and curvature drift. Here w⊥ and w are the perpendicular and parallel particle energies, respectively, while Rc is the radius of curvature and rc points outward from center of curvature. For dipolar fields rc and ∇B are oppositely directed and therefore the gradient and curvature drifts have the same direction, forming an azimuthal drift. Note however that the particle charge enters the azimuthal expression, separating ions (westward motion in a dipolar field) and electrons (eastward motion) and thereby creating a current. Note that close to the Earth, E in the above equation is not only Esw, but composed of Esw and a radially inward directed electric field which is caused by co-rotation of the dense plasma. Hence, there are two distinct convection regions in the magnetosphere; the inner region where plasma is trapped and co-rotates with the Earth, and the outer region where plasma flows Sunward until they come close enough to the dipolar region so that azimuthal drift dominates (see Figure 9).. Figure 9: Magnetospheric convection in the equatorial plane [from Hargreaves, 1995].. 13.

(170) 3 - The Ionosphere. The outermost layer of the Earth's atmosphere consists of ionized gas. The different source (e.g., photo-ionization) and sink (e.g., recombination) mechanisms are effective at different altitudes, giving rise to an altitude profile of the electron density, called the Chapman profile. Two particular regions with high electron densities are called the E (85-140 km altitude) and F (140-ca 1500 km) layers. The electron density exhibits variations during day and season, and also more locally due to e.g. substorm precipitation.. 3.1 The Auroral Oval Aurora is created when charged particles precipitate into the Earth’s ionosphere and collide with neutral atoms, exiting them so that they emit light of certain spectral lines (see Table 2). Due to different energies and interaction mechanisms with the neutral atmosphere, ions and electrons cause aurora in somewhat different regions. For example, typical energies of electron precipitation (up to a few tens of keV) are responsible for the green emission line, which is most dominant in high-latitude aurora. Electron precipitation at lower energies (a few 100 eV) produces red line auroras at higher altitude. Proton aurora is less structured than electron aurora due to interchange between proton - hydrogen, leading to diffusion of the primary particle stream, and the relatively large proton gyro-radius (100−1000 m, compared to the electron's 1−10 m). Prec. particle e− e−. Prec. particle energy (keV) ∼0.1 ∼10. Emission height (km) 230 110. Emitting particle atomic O atomic O. H+. ∼10. 110. atomic H. Emitted line (Å) 6300 (red) 5577 (green) 4681 (Hβ). Table 2: Three auroral emission lines. Precipitating particles and their approximate energies. Approximate emission height, emitting particle, and emitted spectral line.. 14.

(171) When classifying the precipitating particles according to their energy level, three groups with different characteristics appear. Low-energy (< 1 keV) particles arrive from the dayside magnetopause, medium-energy (∼0.5−20 keV) particles are accelerated along the field lines from the tail, and highenergy (> 20 keV) particles leak out from the ring current and plasmasphere. The combined precipitation patterns result in oval-shaped bands around the polar regions. These bands are called the auroral ovals, and mark the border between open (polar cap) and closed field lines. The size of the auroral oval, and the polar cap, vary with geomagnetic conditions, as seen in Figure 10. The oval has a fixed position with respect to the Sun, being somewhat compressed on the dayside. There, on average, it is located at 78° geomagnetic latitude, while on the nightside it extends to about 67°.. Figure 10: The auroral oval for different levels of disturbance, a) quiet conditions, b) medium activity, and c) strong activity [from Starkov and Feldstein, 1967].. 3.2 Ionospheric Currents The current density can be derived from dynamic and electrodynamic equations for charged particles in the ionospheric plasma, and is at a given altitude in the ionosphere given by [notation according to Brekke, 1997]: j = σP E⊥ − σH (E⊥ × B) / B + σ E. 15.

(172) The expressions for the Pedersen, Hall and parallel conductivities are, respectively: σP = ne/B (ke/(1+ke2) + ki /(1+ki2) ) σH = ne/B (ke2 / (1+ke2) − ki2 / (1+ki2) ) σ = ne/B (ke + ki ) Here B is the strength of the magnetic field, e is the electronic charge, and n is the number density of either electrons or ions (charge neutrality is assumed). The fraction of the gyro frequency (Ω) and the charged - neutral particle collision frequency (ν) is represented by the mobility coefficient k=Ω/ν. The subscripts e and i indicate electrons and ions, respectively. As the collision frequency is dependent on the neutral particle number density, the collision frequencies and mobility coefficients will vary with height in the ionosphere. The collision frequency decreases with altitude, while the mobility coefficient has the opposite behavior, increasing with altitude. As seen in Figure 11, σ is the largest of the three conductances, at all altitudes. σP peaks at the altitude ∼120 km, where it also increases above σH.. Figure 11: Altitude profiles of the Pedersen, Hall and parallel conductivities [from Brekke, 1997].. Currents in the ionosphere arise due to separate drift velocities for electrons and ions. E×B-drift is charge- and mass-independent, but collisions with neutral atmospheric particles produces a difference in the effective velocities, and hence a current. 16.

(173) Ion motion At low altitudes in the ionosphere the ions try to E×B-drift but collide with neutral particles. The collisions result in an incomplete gyration around B and an effective movement parallel to the electric field occurs, giving rise to E-drift. At 90 km altitude the ion velocity is basically directed along the Efield. Higher up in the ionosphere the neutral collision frequency decreases and allows the motion of the ions to be more and more dominated by E×Bdrift. The E×B-motion is basically unaffected by collisions above an altitude of 250 km. As the ions are less affected by neutral collisions, their speed increases. Electron motion Electrons do not have the same problem with collisions. They are more tightly bound to the magnetic field lines and have smaller gyroradius, therefore they are practically unaffected by collisions above an altitude of ∼90 km. In fact, for electrons the movement is at all altitudes dominated by E×B-drift.. Figure 12: The electron and ion velocity vectors for three different altitudes [from Brekke, 1997].. 17.

(174) The resulting current density at different altitudes can be studied through the relationship between the ion and electron velocity vectors (Figure 12). The electron velocity vector is roughly constant in the E×B-direction, while the ion velocity vector rotates with height as the ion - neutral collision frequency decreases. The Pedersen current, in the E-direction, is carried by ions, while the Hall current, in the E×B-direction, is carried by electrons. Above ∼140 km the ions and the electrons drift with almost the same velocity, and the resulting current is very low. In summary, the horisontal current density is almost parallel to the electric field in the ionospheric F-region, resulting in a Pedersen current. In the ionospheric E-region and below, the current density is mainly perpendicular to both the electric and magnetic fields, forming a Hall current.. Figure 13: Ionospheric conductivity, electric field, and current structure. The Hall currents form the auroral electrojets and the Pedersen currents close via fieldaligned currents to the magnetosphere [from Kamide and Baumjohann, 1993].. Now we will briefly describe some of the current systems in the ionosphere and how they couple to the magnetosphere. There are sheets of field-aligned currents (FACs), connecting currents at magnetospheric boundaries to the 18.

(175) high-latitude ionosphere. The so-called Region 1 currents map into the crosstail current somewhere between 10−50 Re. They consist of upward FACs from the duskside ionosphere (causing discrete aurora), and downward FACs on the dawnside ionosphere. The so-called Region 2 currents map into the outer part of the ring current and inner edge of the plasma sheet, at 4−7 Re. They consist of upward FACs on dawnside, and downward FACs (causing diffuse aurora) on the duskside ionosphere. The solar wind-induced electric field Esw, which can be seen as another representation of field lines moving (i.e., convection), is mapped down along the field lines, resulting in a polar cap electric field. The convection creates two convection cells with anti-sunward motion across the polar cap and sunward return flow at lower altitudes. The return flow produces new ionospheric electric fields, poleward directed on the dusk side and equatorward directed on the dawn side (top right in Figure 13). These electric fields will drive Pedersen and Hall currents in the ionosphere. The Pedersen currents are directed poleward on the eveningside and equatorward on the morningside, connecting the region 1 and 2 field-aligned currents (bottom right in Figure 13). The convection or auroral electrojets are mainly Hall currents in the ionospheric E-layer, which flow in conductivityenhanced channels in the auroral oval (bottom left in Figure 13). Since they are an effect of ionospheric convection they are said to be electric field driven. The electrojets consist of an eastward directed jet on the duskside, and a westward directed jet on the dawnside high-latitude ionosphere.. 3.3 M-I Coupling and Ionospheric Convection The magnetic field lines connecting the magnetosphere and the ionosphere make it possible for energy exchange between the two regions to occur, hence it is necessary to look at the magnetosphere and the ionosphere as coworking systems that influence each other, in magnetosphere-ionosphere (M-I) coupling. One aspect of this is the convection pattern of the highlatitude ionosphere, shown in Figure 14, resulting from the global magnetospheric convection. The ionospheric convection patterns consist of convection cells with different origin; merging cells, lobe cells, and viscous cells. The roundshaped lobe cell, resulting from lobe merging, is on open field lines within the polar cap, the viscous cells arising from viscous interaction on the flanks, are crescent-shaped on the dawn and dusk edges of the polar cap (closed field lines), and the subsolar merging cells stretch over the polar cap (field lines open on dayside, are convected over the polar cap, close on the nightside, return at lower latitudes). The relative size of these cell patterns is 19.

(176) decided by the solar wind's interaction with the geomagnetic field through IMF By and Bz. During northward IMF the pattern is a combination of lobe and viscous cells. During southward IMF, the convection is mainly driven by subsolar merging, resulting in a two-cell pattern in the ionosphere. In addition, during subsolar merging, the IMF By influence on the process is best visible (see also Figure 8).. Figure 14: Ionospheric convection patterns on the northern hemisphere for various combinations of IMF By and Bz. ‘V’ refers to viscous cells, ‘L’ to lobe cells, ‘R’ to reclosure cells, and finally ‘M’ to merging cells [from Reiff and Burch, 1985].. 20.

(177) 4 - Substorms. The concept of auroral substorms was introduced by Akasofu [1964]. Prior to this aurora was of course studied, and so were the magnetic disturbances measured during active auroral displays. However, now a more global view of the aurora has emerged, as it was realised that the auroral substorm was not a local phenomena but extended over large distances, encompassing the entire magnetosphere. Spacecraft provide global images of the ionosphere and magnetic/particle measurements of the magnetotail, which further prove the large-scale aspect of substorms. Today we know that a substorm is a global magnetospheric disturbance caused by interaction with the solar wind. In a general description, the substorm process is separated into directly driven and loading-unloading parts, which can co-exist during the process. The directly driven part is as the name indicates, driven directly by the solar wind variations and correlates well with the southward IMF component. It manifests itself in increased magnetospheric and ionospheric convection, which e.g. in the high-latitude ionosphere is observable in the enhanced convection electrojets. In energy dissipation terms, the driven process consists of convective dissipation in the ionosphere and ring current. The loading-unloading part manifests itself in storage of magnetic flux and plasma drift kinetic energy in the tail; energy which is later released due to some (yet undetermined) kinetic or MHD instability. The unloading process consists of an energy return to the solar wind and explosive dissipation of energy in the magnetosphere and ionosphere.. 4.1 Observations The substorm process is divided into three phases: the growth, expansion, and recovery phase. Observations which have to be described by substorm theories are described below.. 21.

(178) Figure 15: Growth phase features in the magnetosphere [from McPherron et al., 1973].. Growth Phase The substorm growth phase [McPherron, 1970] is regarded to begin at the southward turning of the IMF [Fairfield and Cahill, 1966]. The association with southward IMF suggests that merging of field lines takes place in front of the magnetosphere. The subsolar merging results in that open magnetic field lines are swept from the dayside, across the polar cap toward the nightside. In this way, flux is added to the magnetotail lobes, and thus the lobe cross-sections increase and storage of magnetic energy is built up. Some magnetospheric features during substorm growth phase are shown in Figure 15. The vertical pressure balance in the tail, between the magnetic pressure in the lobes and the plasma pressure in the plasma sheet, causes the plasma sheet to shrink in its vertical extent. The cross-tail current increases and the tail becomes more 'tail-like', up to a downtail distance of ∼15 Re. The elongated magnetotail and cross-tail current enhancement are linked through Ampère’s law, where a larger curl of the magnetic field equals a stronger current. However, at the same time the cross-section of the tail enlarges further downtail as flux is added to the lobes. During quiet times the current sheet is comparatively weak and far from the Earth, so the transition from dipolar to tail-like fields is gradual. As the plasma sheet thins, the inner edge of the plasma sheet moves Earthward, and 22.

(179) hence also the cross-tail current sheet. Near the inner edge of the plasma sheet (∼8 Re) a region of thin current structure with low-B (a few nT) forms. Another ionospheric signature results from the plasma sheet's inner edge moving closer to Earth as the dipolar field weakens. Disturbance of gyrating trapped particles and pitch-angle diffusion create auroral arcs that move equatorward. Furthermore, the size of the polar cap increases as a measure of the open flux available in the magnetospheric lobes. The enhanced convection across the polar cap is mirrored in enhanced convection electrojets. In summary, the growth phase, which is a slow process lasting up to ∼60 minutes, brings two main magnetospheric implications: the increase of the cross-tail current and of the magnetic flux in the lobes. Before we describe the next phase of a substorm, however, we have to discuss the implications of neutral lines and fast plasma flows, which are observed during substorms. The neutral line(s) When envisioning dayside merging of field lines to be the cause of the increased flux in the lobes and the thinning of the plasma sheet, it is natural to assume nightside reconnection as a process decreasing these parameters. We have already described the distant neutral line (DNL). As the near-Earth plasma sheet thins during the growth phase of the substorm, reconnection starts Earthward of the DNL, forming a new, near-Earth neutral line (NENL). The increasing magnetic pressure due to the reconnected field line and the curvature (tension) force causes the newly reconnected and sharply bent field line to bounce back from the tail, resulting in fast plasma flows from the reconnection line. No direct observations of the NENL have been made, but spacecraft observations of plasma flow (described further in the next section) indicate the neutral line position at a typical downtail distance 20−30 Re [Nagai et al., 1998; Nagai and Machida, 1998; Machida et al., 1999]. Another feature that indicates the presence of the NENL is the observation of travelling compression regions (TCRs). TCRs are seen as bipolar signatures in the Bz-component and compressions of the lobe magnetic field, caused by localized bulges in the plasma sheet that move rapidly (∼300−800 km/s) in the Earthward or tailward direction. The magnetic field caught between the bulging of the plasma sheet and the nearly stationary magnetopause [see Slavin et al., 1993] is compressed and constrained to drape closely about the plasma sheet bulge. TCR studies have indicated the existence of multiple neutral lines during a single substorm [McPherron et al., 1993; Slavin et al., 2002]. It has been suggested that the neutral line is confined in Y-direction [Ohtani et al., 23.

(180) 1999]. Motion of the neutral line is also observed, particularly it is assumed that the near-Earth neutral line draws back in the tail after plasmoid ejection, to form a new distant neutral line.. Figure 16: Reconnection at several neutral lines creates closed flux ropes which, after pinching off all plasma sheet field lines, move either Earthward or tailward, detected as TCRs in the lobe field [from Slavin et al., 2003].. BBFs and flow bursts A neutral line is envisioned to be the cause of plasma flows both Earthward and tailward, and as already mentioned, flows are used to estimate the location of the neutral line. Baumjohann et al. [1989, 1990] investigated the occurrence frequency of fast flows (> 400 km/s) in the CPS and PSBL. They pointed out that the CPS bulk flows are typically single-ion component, as opposed to PSBL flows, which exhibit two counter-streaming ion beams. It has also been shown that the flows are nearly field-aligned in low-β parts of the magnetotail (e.g., PSBL and lobe), whereas in the high-β plasma sheet the flows have a substantial convective (perpendicular to local field) component [Baumjohann et al., 1990; Petrukovich et al., 2001]. This is important, as field-aligned flows cannot transport magnetic flux. Angelopoulos et al. [1992] found flow bursts (velocities up to 1000 km/s) of minute scale within the bursty bulk flows (400−600 km/s) of 10-minute scale. The flow bursts are associated with ion heating and transient field dipolarisations (see next section). The 10-minute time scale flow events are called BBFs to stress their single population, convective bulk flow, as opposed to the PSBL flows. Tailward-moving BBFs are seen predominantly tailward of −19 Re, while Earthward of this distance the BBFs are mainly Earthward-directed [e.g., Baumjohann et al., 1990]. They are most likely to occur close to the midnight meridian. They are mainly directed along Sun-Earth line, but with a small duskward component for both Earthward and tailward flows, consistent with a superposition of a small (∼50 km/s) diamagnetic drift due to the Earthward-directed particle pressure gradient. Prior to, and during the expansion phase, BBFs are observed in the central magnetotail. They are regarded important in transferring flux Earthward 24.

(181) even if they are sporadic. Note however that high-speed flows occur even for low geomagnetic activity, and there is no one-to-one correlation between substorms and BBFs. BBFs are estimated to be localized, with a cross-sectional area up to 10 Re2 in the YZ-plane [Angelopoulos et al., 1996]. BBFs may be responsible for 60−100% of the measured Earthward mass energy and magnetic flux transport past the satellite in the plasma sheet [Angelopoulos et al., 1994]. However, substorm intensities range and there can be numerous BBFs during a substorm. It is also possible that activity is localized in space, and that we therefore might fail observing some localized but persisting phenomena.. Expansion Phase The expansion phase unloads the stored energy in the magnetosphere by a reduction of the cross-tail current and reconnection of lobe field lines decreasing the amount of open flux in lobes. Onset signatures The expansion phase begins with the substorm onset, which has several observable signatures in the ionosphere and in the magnetotail. In the nightside ionosphere the following features are seen: • At auroral breakup the most equatorward arc in the auroral oval suddenly brightens and expands azimuthally as well as poleward to form a bright region [Akasofu, 1964]. The initial brightening usually occurs between 20:00 and 02:00 MLT [Craven and Frank, 1991], and 55° and 75° in geomagnetic latitude [Kamide and Winningham, 1977], which maps to the near-Earth tail at X > −15 Re [e.g., Murphree et al., 1991], i.e., near the inner edge of the cross-tail current. Multiple intensifications of the western part of the auroral region form the westward travelling surge (WTS) of intense aurora, which propagates westward at a velocity of a few km/s [Opgenoorth et al., 1998]. The eastern part of the auroral region develops into an auroral bulge of patchy and diffuse aurora. The auroral features of a substorm are summarized in Figure 17.. 25.

(182) Figure 17: Auroral development during a substorm. A) faint auroral arcs drift equatorward, B) at onset the most equatorward arc brightens, C) the brightened arc expands and forms the WTS, D) continued expansion with morningside activity, E) recovery phase starts, F) the disturbance weakens and returns to initial state [from Akasofu, 1968].. • Pi2 pulsations, i.e., irregular magnetic fluctuations with up to a few nT amplitude and periods of 40−150 seconds. They are usually detected by low- and mid-latitude ground-based magnetometers [for overview, see Olson, 1999]. • A negative magnetic bay [Akasofu and Meng, 1969; Meng and Akasofu, 1969] in the Bx-component, reaching up to 1000 nT. The magnetic bay is the most striking feature of complicated magnetic 26.

(183) disturbances, caused by the superposed westward-directed substorm electrojet (Figure 18), which is the ionospheric part of the SCW system, described next. The substorm electrojet is conductivity-driven, as opposed to the convection electrojets [Kamide and Vickrey, 1983].. Figure 18: The substorm electrojet [from Baumjohann, 1983].. In the magnetotail the following features are seen: • Current disruption, as result of extreme plasma sheet thinning and/or near-Earth reconnection, reduces the cross-tail current locally by forming the substorm current wedge (SCW), with fieldaligned currents down to the ionosphere (see Figure 19). The SCW consists of a downward field-aligned current, carried by ionospheric electrons, on the eastern side, and an upward fieldaligned current, carried by accelerated magnetospheric electrons, on the western side. These field-aligned currents close in the ionosphere by the already mentioned horisontal substorm electrojet. The initial auroral brightening will occur near the ionospheric footprint of the upward directed field-aligned current of the SCW. Prior to the current disruption, the current sheet is deduced to have a thickness comparable to the thermal ion gyroradius [e.g., Lui et al., 1992]. The region of current disruption expands longitudinally [Kokobun and McPherron, 1981; Nagai et al., 1982; Singer et al., 1985] and tailward (at least for r < 15 Re) [Jacquey et al., 1991; Slavin et al., 1997; Ohtani et al., 1999] with a velocity of 200−300 km/s [e.g., Ohtani et al., 1992]. 27.

(184) • Field dipolarisation [McPherron et al., 1973] returns the magnetic field configuration into a more dipolar shape as the cross-tail current decreases (see Figure 19). The region of dipolarised field, i.e. enhanced Bz, can initially be confined in a very narrow sector in the geosynchronous region, but expands azimuthally as well as radially [Ohtani et al., 1991; Lopez et al., 1988]. Dipolarisation has to initiate outside 6.6 Re, as an Earthward expansion of dipolarisation occurs (≤ a few hundred km/s) in the geosynchronous region [Ohtani, 1998].. Figure 19: Above: The SCW reduces the cross-tail current by short-circuiting via field-aligned currents to the ionosphere. Below: Dipolarisation changes the tail’s magnetic field configuration [from Kamide and Baumjohann, 1993].. 28.

(185) • Energetic particle injections [e.g., Belian et al., 1981; Reeves et al., 1997] are observed at geosynchronous orbit (see Figure 20). Tailward of the injection boundary, i.e., in the injection region [Mauk and McIlwain, 1974; Moore et al., 1981; Lopez et al., 1990] electrons and protons are energized together at the same time, i.e., at substorm onset. Electrons and protons can have their injection regions slightly separated in local time, where the particle injection is centered at the auroral breakup rather than midnight longitude [Liou et al., 1999]. The injection is called dispersionless when fluxes at all energies increase simultaneously, which indicates that the observation point is in close vicinity to the onset location. The particles, which have energies ranging between a few tens of keV to hundreds of keV, are subject to energy-dependent gradient and curvature drift, thereby contributing to the ring current. The injection boundary has a radially inward propagation of 24 km/s inside geostationary orbit [Reeves et al., 1996].. Figure 20: The injection boundary behind which energetic particles are injected into the geosynchronous region [from Reeves, 1998].. • Reduction of the lobe field magnitude indicates a decrease in the amount of open flux in the magnetotail. Reconnection of plasma sheet field lines start during the growth phase, and of lobe field lines at substorm onset, hence the peak total field can be regarded as a signature timing the onset in the tail [McPherron et al., 1993].. 29.

(186) • As lobe field lines are beginning to reconnect at the neutral line, a plasmoid is ejected downtail, which might be detected through a tailward propagating TCR (see Figure 16). The plasmoid consists of the pinched-off field lines previously composing the tailward part of the plasma sheet. • Auroral kilometric radiation (AKR) is a radio wave emission in the range 30−800 kHz, which intensifies at substorm onset [Slavin et al., 1993; Murata et al., 1995]. The AKR is generated at ∼1−3 Re, near the auroral field-aligned acceleration regions in the evening and night sectors [Gurnett and Frank, 1973; Green et al., 1979]. The order in which these signatures are seen, in the ionosphere and in the magnetosphere, is of great importance since it can indicate how and where the substorm is initiated. The consensus is that auroral breakup is the first proper onset signature in the ionosphere [see Liou et al., 1998, 1999; Shiokawa et al., 1996, 1998; and references therein for timing comparisons]. There is e.g. a 1−3 minute delay in Pi2s compared to auroral breakup. Dispersionless injections are also typically delayed 1−3 minutes compared to auroral onset. In general, what makes the timing difficult is the instrument location (particularly for spacecraft) relative to the observable feature, which is neither temporally nor spatially stationary. The time resolution also poses a limit, of course. Pseudo-breakups A regular substorm expansion phase lasts ∼30−60 minutes. Smaller activations, so-called pseudo-breakups, have duration of ∼10 minutes and low intensities where the magnetic bay only reaches ≤ 100 nT [Koskinen et al., 1993]. They usually occur before a main substorm onset, however the auroral brightening does not develop into a large-scale auroral bulge but is confined within a few hundred kilometers [Akasofu, 1964]. It is considered that a reconfiguration of the tail field does not occur, even though the trigger mechanism probably is the same as for a proper substorm. Pseudo-breakups should not be mistaken for small substorms; they are more likely quenched, i.e., suppressed or prematurely terminated, substorms. Their study can reveal important isolated initial substorm elements.. Recovery Phase The last substorm phase has traditionally been regarded as a mere return to the ground state of the magnetosphere, where activity slowly decays during ∼60−120 minutes. Recently the recovery phase has begun to be a focus of 30.

(187) research, with the suggestion that this phase exhibits a relocation of the active region in the tail [Pellinen et al., 1992; Opgenoorth et al., 1994]. Activity such as Ps6 pulsations and omega bands are seen in the morning sector in the ionosphere. Ps6 are 10−20-minute magnetic pulsations, caused alternating upward and downward field-aligned currents in the westward electrojet. The pulsations move eastward with a 90° phase shift between east and north components [Pulkkinen et al., 1998]. Omega bands are auroral, wavy structures ranging from hundreds to thousand km, observed at the poleward edge of the diffuse oval. They drift eastward with close to the E×B-velocity (∼1 km/s). The bright auroral regions of omega bands are localized under the upward directed field-aligned currents of Ps6-associated current systems. Pulkkinen et al. [1991] mapped Ps6/omega bands into the post-midnight equatorial plane in the magnetosphere, within a geocentric distance of 10 Re, or even closer Earthward if considering field-aligned currents [Opgenoorth et al., 1994]. The source region for the recovery phase processes is therefore considered to be in the vicinity of the near-Earth plasma sheet.. 4.2 Theories Two main substorm theories suggest different standpoints on the substorm onset cause and location in the magnetotail. Here we try to describe the two models and observations that support and/or contradict them.. Near-Earth Neutral line model The near-Earth neutral line model [Baker et al., 1996; and references therein] relies on merging and reconnection to explain the substorm cycle. The observational evidence we have described in the previous section does indeed point to dayside merging, increased magnetospheric convection, a near-Earth neutral line forming, BBFs transporting flux Earthward, and plasmoid release downtail. Hence, this model places the onset, defined by the reconnection of the last closed plasma sheet field lines, at the location of the near-Earth neutral line, at a downtail distance of 20−30 Re. The Earthward flows create the current disruption and all the other onset features in the near-Earth region. The flux pile-up model [Hesse and Birn, 1991], based on MHD simulations, describes how magnetic flux is convected Earthward from the neutral line. The plasma flow decelerates as magnetic flux is piled up in the region where the magnetic configuration changes from tail-like to dipolar. Observations support this hypothesis, as fast flows are seen less frequently 31.

(188) closer to Earth. As the pile-up proceeds, the region of dipolarisation extends tailward. The tailward motion of the braking point thus corresponds to the poleward expansion of the aurora. Observations show the dipolarisation region moves tailward, consistent with pile-up. However, a sudden increase of cross-tail current, followed by rapid dipolarisation has also been observed at onset. The current increase is usually of less than a minute duration. The spatial scale of the region is harder to determine, ranging between 1 Re or less [Ohtani et al., 1991] or global scale [Slavin et al., 1997]. These observations are not in agreement with flux pile-up, which is described by a monotonically increasing northsouth field component. Another feature which continuous flux transfer does not explain is the explosive character of ground onset phenomena. The BBFs, which bring flux Earthward, are bursty with duration of a few minutes, while the SCW system continues for much longer. Hence, another or intermediate source for the SCW is required. Two sources of field-aligned currents can be considered: 1) azimuthal gradients in both total and plasma pressures toward midnight arise during active times, which can drive a substantial amount of fieldaligned currents [Haerendel, 1992; Shiokawa et al. 1997, 1998b] and 2) shear of the flow velocity at the dawn- and duskside edges of the flow region can also set up field-aligned current systems [Hasegawa, 1979]. The compressional pulses and fluctuations of field-aligned currents generated at the flow braking point might be the cause for Pi2s. As for the energetic particle injections, one theory states that when the Earthward flows are slowed and diverted around the dipolar field, a compressional wave is launched which energizes and injects particles. Another theory suggests that as the SCW is formed the associated enhancement in the electric field injects particles into the geosynchronous region. At the braking point the particles should start drift motion in the dipolar background field, thus making the braking point act as the injection boundary. However, observations show that the injection boundary is usually well inside 10 Re in the midnight sector, while the braking point is probably at 10−20 Re since the occurrence rate of fast flows drops in this range. The order of the various onset signatures, as they are explained by the near-Earth neutral line and pile-up models is shown in Figure 21.. 32.

(189) Figure 21: The order of substorm onset signatures, explained in the context of the near-Earth neutral line and pile-up models [from Shiokawa et al., 1998].. Current disruption model The current disruption model [Lui, 1996] places the onset location nearEarth, at roughly 10 Re downtail, due to some instability reducing the crosstail current. A suggested instability is the cross-field current instability, which takes place in quasi-dipolar field configuration. As the plasma sheet thins near its Earthward edge during substorm growth phase, the cross-tail current sheet thickness decreases to a length scale comparable to the thermal ion gyroradius, and the neutral sheet ions become unmagnetized. Electrons have smaller gyroradius and are more closely tied to the magnetic field lines, hence they remain magnetized. The unmagnetized ions drift across the neutral sheet, and the relative drift between electrons and ions provides energy to excite waves and initiates current disruption, which reduces the current or triggers other instabilities to do this. The current disruption model suggests a diversion of the cross-tail current, i.e. formation of the SCW, as the cross-tail current is prevented from flowing across the tail. Current disruption also leads to particle injection and plasma sheet thickening. Furthermore, a rarefaction wave moves tailward and produces multiple sites of current disruption. The passing of a rarefaction wave leaves a thin current sheet with reduced magnetic field normal to the neutral sheet, suitable for reconnection. Earthward flows in the central plasma sheet are related to convection surges or the transient passing 33.

(190) of rarefaction waves fronts leading to field line collapse with significant perpendicular flow component. The sequence of substorm signatures and their explanations, according to the current disruption model, are shown in Figure 22. Mapping of onset signatures in the ionosphere and in the near-Earth tail suggest that the region around geosynchronous orbit indeed is an important location in the substorm process. For example, auroral break-up, which is the first onset signature seen from ground, maps to the current sheet just outside geosynchronous orbit. In the current disruption model, the brightening of the most equatorward arc is associated with the local reduction of the enhanced tail current, i.e., the instability which triggers the substorm onset.. Figure 22: The order of substorm onset features, explained by the CD model [from Lui, 2000].. Storm - Substorm Relation Geomagnetic storms are magnetic disturbances due to an enhanced ring current around the Earth, resulting in a depression of the H-component, and measured by the Dst-index at relatively low geomagnetic latitudes. Changes in the large-scale dawn-to-dusk convection electric field can trap particles inside geostationary orbit. The electric field is caused by a combination of 34.

(191) solar wind velocity and southward IMF, the latter being most important. Hence, IMF Bz regulates the growth of the ring current. The storm time ring current is carried mainly by energetic ions with energies above several tens of keV. These ions are provided by the solar wind, but they are also emanating from the ionosphere, as atomic oxygen is accelerated from the ionosphere by substorm-associated electric fields. Storms are a consequence of long periods of southward-directed IMF, typically occurring in magnetic clouds in the solar wind, stemming from large-scale coronal mass ejections. Storms are phenomena of extended duration, with a main phase lasting between a few hours and a day, and a recovery phase lasting from hours to tens of hours. The mentioned time durations can however be considerably longer, resulting in storms of several days’ duration. When Akasofu [1964] began studies of substorms, he believed that the optical and magnetic disturbances were part of geomagnetic storms, and considered substorms to be building-blocks for storms, hence the name substorm. Later it proved to be questionable whether ∑ substorms = storm, since substorm activity does not necessarily lead to a storm. As described above, the storm process is also dependent of southward IMF, so storms naturally occur together with substorms. Presently the main question is whether substorm injections contribute to the ring current or not. Substorms inject energized particles, particularly ions, by transporting them to the inner magnetosphere and therefore seem like a natural source for the ring current. However, the Dst-index and substorm injections have not been shown to be correlated, and substorm activity during storm recovery phase has little effect on Dst. The question remains unsolved so far, and for the interested we refer to two major reviews on the standing point of today, regarding storms and their relationship with substorms [Gonzales et al., 1994; Kamide et al., 1998].. 35.

(192) 5 - Instruments and Measurements. In this thesis a number of instruments, both ground-based and satelliteborne, are used. Here we will only make a brief introduction to the groundbased instruments that form the basis of the thesis: magnetometers and radars.. Figure 23: Map over the northern hemisphere. Magnetometers are shown as squares and the fields-of-view of the SuperDARN radars are outlined. The EISCAT radar facility in Tromsö (northern Scandinavia) is marked by a cross.. 5.1 Magnetometers Ground-based magnetometers measure the disturbances of the magnetic field, e.g. during substorms. There is good data coverage from magnetometer networks around the world, of which three high-latitude networks have been 36.

(193) used extensively in this thesis; the International Monitor for Auroral Geomagnetic Effects (IMAGE) in Scandinavia [Viljanen and Häkkinen, 1997], MARIA in Canada [Rostoker et al., 1995], and magnetometers on Greenland, operated by the Danish Meteorological Institute (DMI) [FriisChristensen et al., 1985]. These three networks are pointed out in Figure 23. Magnetometers usually measure three orthogonal components of the field, in coordinate systems previously described, i.e., either (X,Y,Z) or (H,D,Z). It is impossible to separate the true horizontal ionospheric currents, fieldaligned currents, distant currents in the magnetosphere, and currents induced in the Earth’s surface from the measurements alone. For these reasons the ground-based-measured perturbations are usually expressed in terms of equivalent currents, in the framework of a two-dimensional current system. From ground-based measurements magnetic activity indices are derived. The most commonly used for substorm activity include the AU, AL, and AE indices, which are created from a selection of magnetic stations in the auroral zone region. AU is the upper envelope of the perturbation measured at the selected stations, while AL is the lower envelope. AE is simply the difference between the envelopes. A final comment can be made about fast time variations of the magnetic field, which induce an electric field, according to Faraday's law: ∇ × E = − ∂B / ∂t At substorm onset ∂B/∂t associated with the SCW is large, and results in Ohmic currents in the Earth and thereby new induced magnetic fields. In fact, of the measured magnetic deviation interpreted as the SCW, 40% have been shown to result from the ground induction response to substorm onset [Tanskanen et al., 2001].. 5.2 Ionospheric Radars The idea behind ionospheric radars is to emit radio waves, and the echo received from various scattering mechanisms in the ionospheric plasma will provide information about the medium that scattered the signal. This can be done with either a monostatic radar, which has a co-located transmitter and receiver, or with a multistatic radar, which also has passive receivers at other locations than the transmitter. The ionospheric radar measurements fall into two categories. The first category, called coherent scattering, requires large-scale, turbulent variations or irregularities of the ionospheric plasma. The term coherent 37.

(194) means that for a high coherence, there is little change between successive echoes and they can be added (as they have the same amplitude and phase) to improve sensitivity. The second category, called incoherent scattering, uses thermal fluctuations in the plasma density to scatter the signal. For incoherent scattering, successive echoes are not related in amplitude and phase.. Figure 24: Schematic drawing of coherent and incoherent scatter radars and their ionospheric targets. The coherent scatter radar receives information about the fieldaligned plasma irregularities, while the incoherent scatter radar detects the faint signal of thermal fluctuations in plasma density [Figure by Paul Eglitis].. Coherent Scatter Radars Coherent scatter radars receive echoes from density structures in the ionosphere. To obtain the strongest echoes from the field-aligned irregularities, the radar beam pointing direction should be in the plane normal to the direction of the geomagnetic field (see Figure 24). This is possible to achieve in the E-region, using VHF (30-300 MHz) waves. This however requires large distances between radar and ionospheric target, as the ray path is straight. For the F-region it is not possible to achieve normal incidence to the magnetic field, so instead HF (3-30 MHz) waves are used, resulting in the wave being refracted (bent) during the path and therefore meeting the field at right angles in the targeted high-latitude location, where the magnetic field is almost vertical. The Super Dual Auroral Radar Network (SuperDARN) [Greenwald et al., 1995] is a wide network of monostatic HF radars on both hemispheres, typically operating on frequencies between 3-30 MHz. Their northern hemispheric fields-of-view are shown in Figure 23. The measurements are 38.

No results found